>The problem would be just beginning for all the users of terminals and
>terminal emulators that apparently rely on C1 control codes.

Is there a (somewhat) reliable estimate how many such systems are in
use today ?

The only one I know is VMS and various VT2xx, VT3xx etc. terminals
when set into "8-bit Controls" mode. However, both the VMS terminal
driver and the VT series terminals can be set to the "7-bit Controls"
mode in which the C1 control is replaced by a sequence, e.g. <CSI>
becomes <ESC>[ and so on. At least in this environment I don't see why
applications and terminals would have to support C1 control functions.